Algae is a simple, photosynthetic organism that poses a persistent challenge in hydroponic systems. These organisms compete directly with cultivated crops for resources within the nutrient solution. Significant algal growth consumes the dissolved mineral salts intended for the plants, leading to nutrient deficiencies and stunted growth. Algae also deplete the dissolved oxygen in the nutrient solution, particularly at night when they switch from oxygen production to consumption, which stresses the plant roots. Furthermore, a heavy algae bloom can create a slimy biofilm on the system’s surfaces, which can harbor harmful bacteria and clog pumps, tubing, and emitters.
Conditions That Drive Algae Growth
Algae thrive on a combination of light, water, and nutrients, making the typical hydroponic setup an ideal environment for proliferation. The most significant factor driving growth is the exposure of the nutrient solution to light. Even indirect light penetrating reservoir walls or tubing provides enough energy for photosynthesis. The rich, balanced composition of the nutrient solution—specifically nitrates and phosphates—serves as a perfect food source for algae, allowing rapid reproduction. High concentrations of these dissolved salts, which are necessary for crop growth, simultaneously fuel the algae population.
Water temperature also plays a major role, as algae flourish in warmer conditions, generally between 68°F and 86°F (20°C and 30°C). Since the optimal temperature range for many plant roots is slightly cooler, the warmth that promotes root growth can accelerate algal blooms.
Immediate Removal and System Sterilization
Addressing existing algae growth requires a two-pronged approach: physical removal and acute sterilization. For visible algae coating the inside of reservoirs, trays, or tanks, manual removal is the first step. This involves scrubbing the affected areas with a brush or cloth to physically dislodge the slimy material before draining the system completely.
After the physical cleaning, a temporary chemical shock treatment can be applied to sterilize the system and kill any remaining spores. A common and safe agent for this is hydrogen peroxide (H₂O₂), which breaks down into water and oxygen, leaving no harmful residue. For a shock treatment, a concentration of about 10 milliliters of 3% hydrogen peroxide per liter of water can be used to treat the system surfaces, especially while plants are removed or protected. If the plants cannot be removed, a less aggressive dose of 3 to 5 milliliters of 3% hydrogen peroxide per gallon of nutrient solution can be added directly to the reservoir. A complete system flush and replacement of the nutrient solution must follow any strong cleaning or shock treatment. This ensures that the dead algae material and residual chemicals are removed, preventing decomposition and further clogs.
Long-Term Prevention Through Environmental Control
Long-term management of algae focuses on eliminating the environmental factors that encourage its growth, primarily light and warmth. The most effective preventative measure is complete light exclusion from all parts of the water system. This means using only opaque containers, reservoirs, and tubing that block light penetration entirely. Any translucent or light-colored component should be covered, perhaps by painting the exterior surface with a dark color or wrapping it in black plastic or reflective tape to prevent light from reaching the nutrient solution. Even the small amount of light leaking through net pot openings around the plant stem can be blocked by using plant collars or opaque lids.
Controlling the temperature of the nutrient solution also prevents recurrence, as cooler water is less favorable for algal growth. Maintaining the reservoir temperature below 70°F (21°C) can significantly slow the reproduction rate of algae. Methods for achieving this include placing the reservoir in a shaded or cooler area, or employing a water chiller for precise temperature regulation.
Another strategy involves using biological controls by introducing beneficial microbial inoculants, such as certain species of Bacillus bacteria, into the nutrient solution. These microbes compete with the algae for the available nutrients, effectively starving the algae population. For larger or commercial systems, installing an in-line UV sterilization unit can provide continuous control by exposing the circulating nutrient solution to ultraviolet light, which damages the DNA of free-floating algae spores and other microorganisms.